The present invention relates to an apparatus and method for simulating a human mastoid, and, in particular, to an apparatus and method for testing hearing aids and bone conduction testing devices which are of the bone conduction type, so as to ensure that the hearing aid conforms with accepted standards for overcoming the impedance provided by the human mastoid.
Those skilled in the art of hearing aids and related equipment are familiar with the long-felt need to develop a reproducible standard for measuring the ability of a bone conduction hearing device to overcome the impedance of the human mastoid (or other bones in a human skull) and the skin disposed thereon. Additionally, those skilled in the art will recognize that there is also a long-felt need for a device which may be used to ensure compliance with uniform standards which are currently in place.
In many hearing impaired people, portions of the middle ear of been damaged or are otherwise such that simple amplification of the sound is insufficient to enable the person to hear. To overcome this problem, a bone conductive hearing device essentially bypasses the function of the middle ear by propagating vibration to the inner ear via the mastoid or other cranial bone. Thus, the bone conductive hearing device is typically an electromechanical transducer intended to produce the sensation of hearing by vibrating the cranial bones. This is typically done by placing a bone conduction hearing device behind the ear of the user so that a vibrating element of the hearing device rests on the skin which covers the mastoid. Sound waves are converted into vibrational force which is then applied to the skin. The vibrations travel through the skin and the mastoid and are received by the inner ear in a manner similar to that in which the inner ear receives the vibrations of the inner ear in a person with normal hearing.
In order to determine whether bone conductive hearing aids are operating properly, it is necessary to establish a standard of measuring the devices, as well as a testing mechanism for implementing a standard. Additionally, such a standard and mechanism may also be used to test a human mastoid to determine if it functions normally in response to vibratory force. Several approaches have been made in each regard.
In U.S. Pat. No. 3,019,307, a device for measuring reproducible standard for bone conduction receiver measurement is proposed. The standard to which the device is drawn was the result of the National Bureau of Standards and was described in detail in the Journal of the Acoustical Society, November 1955. An electrical equivalent circuit diagram of a machine proposed for testing a hearing device for use on an average mastoid respective to the standard is shown in FIG. 1A. In FIG. 1A, the representation of a bone conduction receiver positioned against a human head includes an inductor, m, which represents the mass of the skin and bone vibrated by the receiver, a resistor, r, which represents the viscous damping due to the skin, and a capacitor, l/k, which represents the compliance or springiness of the skin.
To implement this circuit, a fairly complex, expensive and bulky machine was used. A cross-sectional view of one embodiment of the machine is provided in FIG. 1B. A bone conduction vibrator (14) is placed on a magnesium disk (10) which is supported by one or more arms (16). When the bone conduction vibrator is turned on, force is transferred through a piston block (22) and measured by an accelerometer (50). Damping of the disk (10) so as to simulate the skin, is provided by an air space (32) between the disk and the piston (30).
Additional research was performed and mechanical impedance values for an idealized average cranial bone (either mastoid or other) were created by the International Organization for Standardization prior to 1970, and were incorporated into the ANSI S3.13-1972 (R-1977), American National Standard for an Artificial Headbone for the Calibration of Audiometer Bone Vibrators and into IEC Publication 373 (1971). An approximate equivalent circuit for the artificial idealized headbone is shown in FIG. 1C, wherein m is a mass of 0.77.times.10.sup.-3 kg, r is 19.3 Nsm.sup.-1, and k is 2.25 .times.10.sup.5 Nm.sup.-1. The goal of the equivalent circuit was to provide a testing device which could replicate the impedance of an average headbone as shown in Table I.
TABLE I ______________________________________ Mechanical Mechanical Mechanical Frequency reactance resistance impedance (Hz) N s m.sup.-1 N s m.sup.-1 N s m.sup.-1 ______________________________________ 125 -290.0 74 299 160 -220.0 55 227 200 -180.0 44 185 250 -140.0 36 145 315 -110.0 29 114 400 -89.0 25 92 500 -71.0 22 74 630 -55.0 20 59 800 -42.0 19 46 1000 -32.0 18 37 1250 -23.0 17 29 1500 -17.0 17 24 1600 -15.0 17 23 2000 -8.4 17 19 2500 -2.2 18 18 3000 +2.7 18 18 3150 +3.9 18 18 4000 +10.0 19 21 5000 +17.0 21 27 6000 +22.0 23 32 ______________________________________
In ANSI S3.13-1987, 8 kHz was added as a test frequency.
In ANSI S3.26-1981, however, it was noted that no commercial product had become available that matches the impedance values within close tolerances, and that some of the devices attempting to match the values were inconsistent. Because no testing apparatus was available that met the standard, an appendix to ANSI S3.26-1981 set forth a type 4930 artificial mastoid as being the testing apparatus of choice, apparently because it was the most accurate device available. A partial cross-sectional view is shown in FIG. 1D. The device includes a loading mass (60) which is sandwiched between a butyl rubber cover (62), and a neoprene disk (64). The two rest on a domed based (66) which is in turn positioned above guide pins (68), ceramic disks (70) and a central electrode (72) which is connected to an output (74). An inertial mass (76) is also provided.
Those skilled in the art will be familiar with the device. The device is relatively expensive and is difficult to calibrate. In order to ensure an accurate result, numerous springs must be adjusted. Because of these factors, many hearing specialists do not purchase a device for conducting tests on bone conduction hearing devices. Rather, they simply purchase an artificial ear, a device for calibrating earphones used in hearing tests.
A side cross-sectional view of an artificial ear is shown in FIG. 1E. The artificial ear is made to internationally accepted specifications, and includes a generally cylindrical housing (80) with an opening (82) at one end. A small vent hole (84) is provided in the housing, along with a hole (86) for receiving the chord of a precision microphone (88). The volume of a void (90) between the opening (82) and the microphone (88) is six cubic centimeters, the average volume of air in a human ear.
Because of the wide spread availability of the artificial ear, and the fact that it is considerably less expensive than the artificial mastoids, it would be beneficial to find an apparatus and method which would allow an artificial ear to be used to test bone conduction hearing devices. Additionally, it would be beneficial if the apparatus and method was as accurate, or more accurate in representing the impedance of a human mastoid than the devices of the prior art.